Usually, in such an electric trolley conveyor the rail has a double-T cross section and the electric trolleys engage the lower flange with two drive wheels at both sides of the middle web of the rail. With such a type of an electric trolley conveyor the goal of small overall height of the electric trolleys cannot be fully satisfied.
As long as such an electric trolley conveyor is only designed for horizontal movement, the demands on the guidance by the rail are not very critical. However, if the trolley conveyor is also to be operated with inclining and declining rail sections, such sections must be provided with sprocket chains or racks which cooperate with a sprocket wheel or a pinion, which in turn is mounted coaxially with the drive wheel and driven jointly with the drive wheel. Counter pressure rollers are necessary in order to secure a sufficient engagement between the sprocket wheel or the pinion and the chain or rack, respectively, especially if the inclining and declining rail sections are steep. The counter pressure rolls must engage the rail in such a fashion, that the drive wheels are always pressed against the corresponding surface of the rail. Further, lateral support rolls are generally necessary. With trolleys of this kind the positioning of the bus bars and of the associated electric supply cables is difficult.
For this reason, the electric trolleys are usually built as upper chord trolleys as shown for example in German patent application 25 45 907, even though this imparts a greater height to the electric trolleys. In this case the drive wheels engage the upper surface of the upper flange or the upper chord of the rail while the counter pressure rolls engage the lower surface of the lower flange or chord of the rail. Therefore, the rail must be connected to the support structure through rigid brackets. Further, it is known to use lower chord electric trolleys in conjunction with a rail with an upright U-cross section which opens toward the side. In this case the drive wheel engages the upper surface of the lower chord while the counter pressure rolls engage the lower surface of the upper chord. The electric trolley is wrapped around the upper side of the rail so that lateral support rolls can engage the side of the vertical web of the rail facing away from the drive wheel. Again, brackets for supporting the rail are necessary. Further, the bus bar must be connected with the lower side of the lower flange so that the free movement of the electric trolleys is not hindered by the power supply cables. This requires an additional increase of the overall height of the trolleys (German patent application 27 50 453).